Deconstructing the spectrum of the soft X-ray background

The soft X-ray background in the 0.1-1.0 keV band is known to be produced b y at least three sources: the Local Hot Bubble (LHB), the extragalactic pow er law (EPL), and a seemingly Galactic component that lies outside the bulk of the absorption that is caused by the ISM of the Galactic disk. This las t component, which we call the "transabsorption emission" (TAE), has been m odeled by a number of groups who have derived disparate measures of its tem perature. The differences have arisen from differing assumptions about the structure of the emitting gas and unrecognized methodological difficulties. In particular, spectral fitting methods do not uniquely separate the TAE f rom the foreground emission that is caused by the LHB. This " degeneracy" c an be resolved using the angular variation of the absorption of the TAE. We show that the TAE cannot be characterized by a single thermal component; n o single component model can be consistent with both the spectral energy di stribution of the TAE and the angular variation caused by absorption by the Galactic disk. We use the angular anticorrelation of the ROSAT All-Sky Sur vey (RASS) with the Galactic absorption to separate local from distant emis sion components and to fit the spectral energy distribution of the resultin g distant emission. We find that the emission is best described by a two-th ermal-component model with log T-S= 6.06+/-(0.19)(0.20) and log T-H= 6.46+/ -(0.12)(0.08). This two-thermal-component TAE fits the ROSAT spectral energ y distribution significantly better than single-component models and is con sistent with both angular variation and spectral constraints.